Differential absorption of electromagnetic radiation by the atmosphere is well known and has been used in various past systems. U.S. Pat. No. 3,103,586, for example, discloses a system to determine the distance from an object at elevated temperature by sensing its infrared radiation, comparing the strength of the signal at the frequency characteristic of the absorption of infrared radiation by H.sub.2 O or CO.sub.2 with the strength of the signals at frequencies adjacent the frequency characteristic of such absorption by CO.sub.2 or H.sub.2 O, and determining from the fraction of energy absorbed by H.sub.2 O or CO.sub.2, from non-linear information on the relationship of CO.sub.2 density and altitude and from the gas pressure or effective gas pressure, the distance from the hot object to the detecting system. The system of U.S. Pat. No. 3,103,586 depends upon a complex electromechanical system that determines distance from the mechanical positioning of optical elements such as interference filters and glass, clay or comb filters and from various stored non-linear data.
U.S. Pat. No. 3,803,595 discloses a system for exciting various atmospheric pollutants with electromagnetic radiation, and detecting and locating such pollutants by their characteristic electromagnetic radiation as a result of their excitation.
U.S. Pat. No. 4,277,170 discloses an aircraft proximity warning and collision avoidance device in which a first aircraft carries a laser beacon and a second aircraft carries an optical detection device designed to alert the pilot of a potential collision threat. To provide discrimination of a signal from background light and noise, the system of U.S. Pat. No. 4,277,170 provides a spectral width on the order of 1/millionth of the visual spectrum, collimates the laser beam, provides the laser beam with a known single polarization, sweeps the collimated laser beam through 360.degree. of horizon and provides a plurality of detectors on the second aircraft. The system distinguishes signals by a known scanning rate of laser. The distance between the laser beacon and second aircraft is determined from the known scanning rate of the laser beacon, the distance between a plurality of detectors on the receiving aircraft and the time delay between detection of the scanning laser beam by the plurality of detectors.
U.S. Pat. No. 4,755,818 discloses an aircraft collision warning system employing a low power omnidirectional pulsed laser source in one aircraft and an omnidirectional laser receiver in another aircraft which operates a light or audio device in the event it detects the laser output of another aircraft.